Here, the union's constructor is defined to activate its empty data
member _M_rest, but during constexpr evaluation of this constructor the
subobject constructor call to O::O(&_M_rest, 42) produces no side
effects that actually activates the member, so the union still appears
uninitialized after the fact. This patch fixes this by faking up a
dummy MODIFY_EXPR in this situation, whose evaluation ensures the member
gets activated.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK
for trunk?
PR c++/102163
gcc/cp/ChangeLog:
* constexpr.c (cxx_eval_call_expression): After evaluating a
constructor call for an empty union member, produce a side
effect that makes sure the member is activated.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/constexpr-empty17.C: New test.
---
gcc/cp/constexpr.c | 34 +++++++++++++++----
.../g++.dg/cpp0x/constexpr-empty17.C | 21 ++++++++++++
2 files changed, 49 insertions(+), 6 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 7772fe62d95..40b0b80b438 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -2787,12 +2787,34 @@ cxx_eval_call_expression (const constexpr_ctx *ctx,
tree t,
&jump_target);
if (DECL_CONSTRUCTOR_P (fun))
- /* This can be null for a subobject constructor call, in
- which case what we care about is the initialization
- side-effects rather than the value. We could get at the
- value by evaluating *this, but we don't bother; there's
- no need to put such a call in the hash table. */
- result = lval ? ctx->object : ctx->ctor;
+ {
+ /* This can be null for a subobject constructor call, in
+ which case what we care about is the initialization
+ side-effects rather than the value. We could get at the
+ value by evaluating *this, but we don't bother; there's
+ no need to put such a call in the hash table. */
+ result = lval ? ctx->object : ctx->ctor;
+
+ if (!result && new_obj
+ && TREE_CODE (new_obj) == COMPONENT_REF
+ && TREE_CODE (TREE_TYPE
+ (TREE_OPERAND (new_obj, 0))) == UNION_TYPE
+ && is_really_empty_class (TREE_TYPE (new_obj),
+ /*ignore_vptr*/false))
+ {
+ /* This constructor call for an empty union member might not
+ have produced a side effect that actually activated the
+ member. So produce such a side effect now to ensure the
+ union appears initialized. */
+ tree activate = build2 (MODIFY_EXPR, TREE_TYPE (new_obj),
+ new_obj,
+ build_constructor (TREE_TYPE
(new_obj),
+ NULL));
+ cxx_eval_constant_expression (ctx, activate, lval,
+ non_constant_p, overflow_p);
+ ggc_free (activate);
+ }
+ }
else if (VOID_TYPE_P (TREE_TYPE (res)))
result = void_node;
else
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
new file mode 100644
index 00000000000..9d753a3bb69
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty17.C
@@ -0,0 +1,21 @@
+// PR c++/102163
+// { dg-do compile { target c++11 } }
+
+struct O {
+ constexpr O(int) { }
+};
+
+union _Variadic_union {
+ constexpr _Variadic_union(int __arg) : _M_rest(__arg) { }
+
+ int _M_first;
+ O _M_rest;
+};
+
+
+struct _Variant_storage {
+ constexpr _Variant_storage() : _M_u(42) {}
+ _Variadic_union _M_u;
+};
+
+constexpr _Variant_storage w;
--
2.33.0.328.g8b7c11b866
